1. Field of the Invention
The present invention relates to a method for safely and securely storing radiocontaminated wastes, such as soil, sludge generated from treatment of waste water and sewage, boiler ashes, rubbles from manmade and/or natural disasters, farmed and/or forest mushrooms, and leaves and also relates to the container for the above method.
2. Description of the Related Art
According to the U.S. classification system, nuclear waste is classified into high-level waste (HLW), transuranic (TRU) waste, uranium mill tailings, and low-level waste (LLW). Generally, not less than 99% of the total radioactivity in nuclear waste is contained in HLW, while LLW takes up the biggest share, about 85% or more of the entire weight of nuclear waste generated. Of the above, soil, sludge, boiler ashes, rubbles, forest mushrooms, fallen leaves, and the like (hereinafter referred to as “radiocontaminated waste matter”) are belonged to LLW.
The general practice for decontaminating radiocontaminated agricultural lands, roads, school lands, and the like resulted from a nuclear accident is to remove the contaminated surface soil and for private houses and public buildings is to wash their roofs using jet water. The said removed radiocontaminated waste matter is then shielded by storing in, for example, flexible container bags, sandbags or the like and then isolated.
The Japanese Environment Ministry has proposed temporary storage sites for smoothly storing and managing a large amount of radiocontaminated waste matter (Patent document 1). In those temporary storage sites, flexible container bags, sandbags, and the like, each filled with radiocontaminated waste matter, are stacked on a water-barrier sheet laid on the ground. These flexible container bags, sandbags are, in turn, shielded by placing filling soil and sandbags upon them.
However, since the said shielding filling soil and sandbags are susceptible to damage from rain, snow, earthquakes, and other causes, and because the radiocontaminated wastes-containing flexible container bags and sandbags have low densities and thickness, moisture from the surrounding ground may permeate the water-barrier sheet, and hence, radiocontaminated waste matter may, in turn, contaminate the surrounding environment (air, groundwater, and the like) of the temporary storage site.
In order to improve the temporary storage site of the type described above, storage facilities for radiocontaminated materials disclosed in the Patent document 1 have been proposed as alternative facilities. In the proposed storage facilities, radiocontaminated materials are shielded by a dome-shaped structure made of corrugated steel sheets and buried in healthy soil. For these storage facilities, the thickness of the corrugated steel sheet is 0.6 cm and the shallowest depth of the healthy soil 30 cm. It is calculated that these values allow the radiation dosage to be reduced by 90%. In this calculation, the half value layers (HVL) of iron and soil are assumed to be 1.5 cm and 5 cm, respectively, and these HVL's are applied only to 137Cs. Since the radiation dosage from radiocontaminated materials is due to 134Cs, 137Cs, 60Co, and the like in the gamma ray, the accuracy of the said calculation remains uncertain.
In a storage structure for shielding radioactive substances-containing materials disclosed in the Patent document 2, bags filled with contaminated soil are placed on a bottom water barrier layer (sheet), and a radiation shielding wall structure is constructed composed of stacked sandbags or retaining walls around its periphery. The topmost surfaces of the bags are covered with a covering body made of a radioactive cesium absorbing powder containing one or more of zeolite, lead, tungsten, barium sulfate, and the like, and a resin or a rubber blended therewith. In order to keep out water, the storage structure is covered with a tent roof. However, the shielding effect of the covering body of this storage structure against gamma rays and the specifications thereof are not disclosed.
In both the storage facilities for radiocontaminated materials (Patent document 1) and the storage structure for radioactive substances-containing materials (Patent document 2), radiocontaminated soil is filled into a flexible container bag and/or a sandbag, neither of which can be shielded against gamma rays. However, in a radiation shielding building disclosed in the following Patent document 3, a mixture of highly functional ceramic concrete and construction materials (such as wood, iron, and concrete) is used without incorporating ordinary lead, lead alloy, antimony-containing material, or the like. In one embodiment, when radiocontaminated soil was filled into a first box made of a highly functional ceramic concrete having a thickness of 5 cm, the radiation dose decreased from 147 μSv/h to 7.5 μSv/h, and a shielding rate of 94.9% was obtained, corresponding to a half value layer of 1.165 cm of the highly functional ceramic concrete. When a second box made of a highly functional ceramic concrete having a thickness of 5 cm was provided around the first box, the radiation dose further decreased from 7.5 μSv/h to 2.0 μSv/h, and a shielding rate of 73.3% was obtained, corresponding to a half value layer of 2.622 cm. Furthermore, when a third box made of a highly functional ceramic concrete having a thickness of 10 cm was provided around the second box, the radiation dose decreased from 2 μSv/h to 0.9 μSv/h, and a shielding rate of 55% was obtained, corresponding to a half value layer of 8.681 cm. The final radiation dose of 0.9 μSv/h is still higher than the 0.065 to 0.072 μSv/h radiation dosage in Fujinomiya city, Shizuoka prefecture, at a straight-line distance of approximately 330 km from the earthquake- and tsunami-damaged nuclear power plant in Fukushima Prefecture. Since the half value layer of the highly functional ceramic concrete changed with varied radiation dosages, it seems likely that the density was uneven and/or the shielding efficiency decreased at low radiation dosages. The following equation 1 was used to calculate the half value layer (Patent document 1).1−Shielding rate=1/[e(thickness of shielding structure+half value layer of shielding structure×ln2)]  [Equation 1]
Additionally, a radiation shielding material disclosed in the Patent document 4 is made by granulating or molding a water slurry containing magnesium oxide and debris of a lead-containing glass, such as discarded cathode-ray tube glass, followed by drying. Although lead-containing glass can be formed as a plate having a thickness of 1 to 10 cm, the performance (half value layer), density, Mohs hardness, and the like thereof are not disclosed.
This applicant carried out a decontamination field test for a radiocontaminated rice paddy using a paper sludge-derived sintered carbonized porous grains and obtained the results indicating that radioactive substances could be removed from radiocontaminated agricultural soil by this method. Furthermore, it was found that the polished rice harvested from the improved soil contained a total of 30 Bq/kg of 134Csc and 137Cs, which is lower than the new Japanese standard limits of 100 Bq/kg for radiocesium in foods. Details are disclosed in the Patent document 5 below.
The said paper sludge-derived sintered carbonized porous grains are formed by sintering and carbonization of paper sludge discharged from paper manufacturing mills which use either waste paper or wood chip or both waste paper and wood chip and the composition thereof is as described below.
(1) Paper sludge discharged from paper manufacturing mills which use either waste paper or wood chip or both waste paper and wood chip is processed by sintering/carbonization to form a paper sludge-derived sintered carbonized porous grains which have a pH of 8 or more and preferably 10 or more; an alkalinity equivalent value of 1.0 to 4.0 meq/g (as NaOH) and preferably 1.5 to 2.5 meq/g (as NaOH); a cation exchange capacity of 1.0 to 4.0 meq/100 g (as NH4+) and preferably 1.5 to 3.0 meq/100 g (as NH4+); an electric conductivity of 70 to 150 μS/cm; a sodium content of 0.0003% or more; and a potassium content of 0.0003% or more, and the paper sludge-derived sintered carbonized porous grains thus obtained is dispersed on or mixed with radiocontaminated soil to remove radioactive substances therefrom.
(2) In the manufacturing process of the said paper sludge-derived sintered carbonized porous grains, the impregnation of the paper sludge with either potassium iodide (KI) alone or ethylenediaminetetraacetic acid (EDTA) alone or a combination of KI and EDTA was not incorporated.
(3) The radiocontaminated soil contains radioactive 134Cs and 137Cs at a total dosage of 800 Bq/kg or above.
(4) The dosage of the said paper sludge-derived sintered carbonized porous grains spread on or mixed with the radiocontaminated soil is 0.1 to 6 kg/m2 (0.5 to 50 kg/m3) (0.1 to 6 percent by weight of dry soil) and preferably 1.0 to 3.5 kg/m2 (8 to 30 kg/m3) (0.9 to 3.3 percent by weight of dry soil).
(5) The paper sludge has a moisture content of 50% to 85%, and after being pelletized and dried, this paper sludge is pyrolyzed in a reducing carbonization sintering furnace at a temperature of 500° C. to 1,300° C., preferably 700° C. to 1,200° C. Furthermore, carbonization is preferably carried out at 800° C. to 1,100° C.
(6) The said paper sludge-derived sintered carbonized porous grains contain, on oven-dry weight basis, 15% to 25% of combustibles (including carbon), 0.5% to 3.0% of TiO2, 0.0001% to 0.0005% of Na2O, 0.0001% to 0.0005% of K2O, 15% to 35% of SiO2, 8% to 20% of Al2O3, 5% to 15% of Fe2O3, 15% to 30% of CaO, 1% to 8% of MgO, and a balance of 0.5% to 3.0% (including impurities), the total of these being 100%; and has a water absorption rate of 100% to 160% in accordance with JIS C2141, a specific surface area of 80 to 150 m2/g in accordance with the BET adsorption method, and an interconnected cell structure.
(7) The said paper sludge-derived sintered carbonized porous grains are to have a porosity volume of not less than 70%, a porosity volume of not less than 1,000 mm3/g, an average pore radius of 20 to 60 μm, and pores with radius of not less than 1 μm constitute not less than 70% of the total porosity volume, and are a mixture of various forms such as spherical, oval, or cylindrical or the like forms with each having an axis length of 1 to 10 mm, and a black color.